This invention relates to the monicoring of fluent materials such as liquids within a vessel, such as a tank, of any size or shape, in order to determine the liquid volume, level or flow rate, measured as the change in volume as a function of time, to or from the tank.
The prior art teaches numerous methods and systems for determining the quantity of liquid or liquid-like materials in a vessel on a continuous, intermittent or occasional basis. Included among such prior art monitoring systems are apparatus for determining liquid level in a tank utilizing, for example, a surface level float, an echo ranging device through which signals are reflected from the surface of the liquid being monitored and electromechanical sensors. In addition to acoustical types of devices utilized for monitoring liquid quantity within a tank, electrical devices have also been devised such as those based on the measurement of electrical capacitance.
One serious problem associated with prior art liquid quantity monitoring systems arises from changes in the orientation of the liquid enclosed within the vessel which can be the result of the vessel being mounted on a moving vehicle whereupon it is subject to acceleration forces. In this case the orientation of the liquid within the vessel to the position of the sensor may vary considerably with a resultant decrease in accuracy.
In an attempt to cope with such problems, acoustical devices utilizing the Helmholtz resonator principle have been proposed, as disclosed for example, in U.S. Pat. Nos. 2,666,326; 3,075,382 and 3,324,716. Generally, the acoustical devices based on the Helmholtz resonator principle involve a tank configuration in which a main cavity is formed within which a liquid body is retained below an air space in communication with a restricted throat passage through which acoustical exciting energy is transmitted from an acoustical source and through which the resulting vibrations are sensed. The geometry of such a resonator configuration determines a cavity resonance frequency for the exciting acoustical energy at which resonance occurs as detected by a microphone mounted in the through passage as explained for example in U.S. Pat. No. 3,324,716 to Roberts aforementioned.
The use of the Helmholtz resonator principle in accordance with the prior art exemplified by the aforementioned prior U.S. Patents, requires a comparison between the sensor outputs of a standard resonator container excited at its known resonance frequency and, a container enclosing an unknown quantity of liquid in order to determine the liquid volume. According to the aforementioned Roberts patent, adjustment of the tank volume is effected until the standard Helmholtz resonator dimension is achieved. Such applications of the Helmholtz resonator principle render the prior art liquid volume monitoring systems relatively limited with respect to installation and application and often inaccurate.
U.S. Pat. No. 4,811,595, incorporated herein by reference, teaches an advantageous utilization of the Helmholtz resonance principal for the purpose of determining the volume of a container. This patent teaches varying the acoustical energy output of a source, e.g. a loudspeaker, in frequency between limits to sweep a frequency band encompassing two cavity conditions which depend on the quantity of a fluent material within a tank that is excited by such acoustical energy. The volume of the fluent material is calculated from the excitation frequency registered during verified detection of resonance conditions, based on abrupt changes in signal characteristics and stored data relating to the geometry of the acoustic sensor arrangement through which the tank interior is monitored. Although this method is very advantageous, particularly when compared to prior art methods, one potential disadvantage is that, among the wide range of frequencies utilized, there may be some that can cause problems to the user, in that they may result in undesired resonance, such as column resonances between the loudspeaker and the surface of the liquid, being realized.